Hybrid vehicles having both an engine and a motor as power source have been proposed recently. The hybrid vehicles are divided into two large groups: series hybrid vehicles and parallel hybrid vehicles. In the series hybrid vehicle, a generator converts all the output power of the engine into electric power and a motor linked with a drive shaft is driven with the electric power. In the parallel hybrid vehicle, the output power of the engine is divided into two portions. One portion is transmitted directly to the drive shaft in the form of mechanical power, whereas the residual portion is converted into electric power and output to the drive shaft.
The series hybrid vehicle enables the engine to be driven at a drive point of the highest driving efficiency, which is selected among possible drive points that allow supply of the electric power required for driving the motor linked with the drive shaft. The series hybrid vehicle can thus be driven with the high driving efficiency, irrespective of the driving conditions of the vehicle.
The parallel hybrid vehicle enables part of the power output from the engine to be transmitted directly to the drive shaft in the form of mechanical power. There is naturally no loss due to the conversion into electric power. The parallel hybrid vehicle accordingly attains the higher driving efficiency than the series hybrid vehicle. The parallel hybrid vehicle, on the other hand, requires the complicated control procedures to drive the vehicle with the high driving efficiency, compared with the series hybrid vehicle. The loss in the course of the power transmission may be increased according to the driving conditions of the vehicle and lower the driving efficiency.
The prior art hybrid vehicle is fixed to either the series hybrid structure or the parallel hybrid structure. No discussion has been made on the technique of simultaneously utilizing the advantages of both the series hybrid structure and the parallel hybrid structure. There is accordingly a requirement for further improvement in driving efficiency of the prior art hybrid vehicle. This issue arises not only in the vehicles but is commonly found in any hybrid-type power output apparatus.